Conventional radiographic imaging, especially for use in medical diagnostics, has traditionally used silver halide based photographic films to capture shadow cast, or latent images of an object. Since silver halide has limited sensitivity to x-ray radiation, phosphor screens are typically used in conjunction with the photographic films in order to reduce the radiation dosage, which is of particular concern when the object being exposed is a person or animal. The films require a light-controlled environment throughout the film-handling process. The use of photographic film involves wet processing chemistry, the disposal of which has been identified as a significant environmental concern.
Electronic radiographic systems based on phosphor screens depend on detecting light emitted by the stimulable phosphor when externally stimulated by either heat or light. The emitted light is captured by a light detector, such as a photomultiplier tube, and is converted to electrical signals representative of the latent image.
Several U.S. patents describe a variety of read-out systems for photostimulable phosphor screens. U.S. Pat. No. 5,180,915 (Ohgoda; Radiation Image Information Reading Apparatus; issued Jan. 19, 1993) discloses a system involving phosphor stimulation by actinic radiation. A reader simultaneously performs reading and erasing processes. The reader extracts a screen from a cassette, reads the image, erases the image and reloads a new screen into the cassette.
U.S. Pat. No. 4,931,641 (Ohgoda; Radiation Read-out Apparatus; issued Jun. 5, 1990) discloses a system involving phosphor stimulation by actinic radiation. A reader extracts a screen from a cassette or magazine, reads the image, and reloads the cassette or magazine.
U.S. Pat. No. 4,893,011 (Bauer; Method of Processing X-Ray Film Cassettes With Phosphorus-Coated Films And A Reading Station For Executing The Process) discloses a system involving phosphor stimulation by actinic radiation. A reader extracts a screen, reads the image on the screen with a laser, erases the screen, and re-inserts the screen back into the cassette.
U.S. Pat. No. Re. 31,847 (Lucky; Apparatus And Method For Producing Images-Corresponding To Patterns of High Energy Radiation; issued Mar. 12, 1985) discloses a system involving phosphor stimulation by heat.
A problem associated with these approaches is the limitations of the phosphor screens used. It is known that such intensifying screens, while increasing the amount of radiation available for detection, also have the effect of reducing the sharpness of the resultant image. In general, image distortion in luminescent screens or structures is caused by the diffusion of light within the luminescent material which causes a blurring of the image with consequent loss of definition and contrast. This diffusion of light is brought about by two fundamental physical processes. First, as the ionizing radiation is converted into light, the direction of the emission of light is random so that it is emitted approximately equally in all directions. The second effect is that the high energy radiation is penetrating, and the depth of penetration is dependent upon the energy of the impinging radiation and the nature of the-material being penetrated. The higher the energy, the deeper the penetration. A lower density material will also lead to a deeper penetration. As a result, the depth of penetration dictates the required thickness of the screen.
Thus, it is seen that as visible light is generated along a path through the screen and normal to its surface, light will be radiating in all directions. Some of the light radiated at an angle off the normal to the surface of the screen will reach the detecting means and result in a diffuse image.
As a result, the design of such intensifying screens has involved a trade-off between screens of large thickness, which result in increased luminescent radiation for a given x-ray level, but which also produce decreased image sharpness, and screens of less thickness, which result in improved image sharpness relative to the thicker screens, but which also require more x-ray radiation to produce acceptable images, thereby increasing the x-ray dosage to which the patient must be exposed.
Thicker or high speed screens are utilized in those applications which do not require maximum image sharpness, thereby reducing the patient exposure to x-rays, while medium speed and slow speed screens are utilized when increased image resolution is required. These latter screens employ thinner phosphor layers and may incorporate dyes to minimize transverse propagation of light by attenuating such rays more than a normal ray which travels a shorter path. In general, detail or slow speed screens require a significantly higher x-ray dosage than the high speed screens.
To address this phosphor-related problem, manufacturers developed non-phosphor media. Unlike the phosphor systems described above, detector systems based on electrostatic imaging convert a latent image generated by exposure to x-ray radiation to an electrical signal. This is done by measuring charge flow differences which reflect the surface charge distribution when irradiated with externally applied actinic radiation.
One such media and the system in which it is used is described within U.S. Pat. No. 3,650,620 (Hoyt, III; Automated Xerographic Processing System; issued Mar. 21, 1972). This patent describes an automated system for light-controlled extraction of a electrically charged xerographic imaging plate from a cassette, reading the latent image and re-insertion of the imaging plate back into the cassette. A problem with this approach and those noted above is the requirement for special feeding devices for extracting the recording medium from the cassette and returning the recording medium back into the cassette. Special care must also be taken not to damage the surface of the recording medium during handling.
U.S. Pat. Nos. 4,176,275 (Korn et al.; Radiation Imaging and Readout System And Method Utilizing A Multi-layered Device Having A Photoconductive Insulative Layer; issued Nov. 27, 1979) and U.S. Pat. No. 5,268,569 (Nelson et al.; Imaging System Having Optimized Electrode Geometry and Processing; issued Dec. 7, 1993) disclose a photoreceptive plate which addresses the resolution limitation of the storage phosphor imaging systems. Another similar disclosure is found in U.S. patent application Ser. No. 08/163,266 (Nelson et al.; Imaging System Having Optimized Electrode Geometry and Processing; filed Dec. 6, 1993). However, being photosensitive, this photoreceptive plate must not be subjected to ambient radiation. Furthermore, this photosensitive plate can be fragile and require care when handled during imaging and read-out. Moreover, the construction of this photoreceptive plate has made the read-out of the latent image difficult.